BGE: Can a cube copy the rotation of another cube in game?
This is possible with a simple python script:

import bge

def main():
    scene = bge.logic.getCurrentScene()

    cube1 = scene.objects["realCube"]
    cube2 = scene.objects["secondCube"]

    cube2.worldOrientation = cube1.worldOrientation


where realCube and secondCube are the names of the actual cube objects in blender.



To attach this script to secondCube, you can use a python controller with an always sensor. Note that the script name must end in a .py extension for the script to be recognized as a module, and that the python controller is given a method (.main) defined as a function inside the script: